PBT Gets a Makeover

With engineering plastics, going around in circles may turn out to be a good thing. A new cyclic form of polybutylene teraphthalate (PBT) promises to open up new design possibilities for molded parts in automotive, industrial, and consumer applications.

Engineering thermoplastics, including PBT, normally have a long chain-like arrangement of their molecules, but scientists at Cyclics Inc. have reinvented the structure of this popular engineering material. They've developed a patented way to break apart PBT's long chains and then reconstitute the molecules in smaller, circular groupings. The company calls this cyclic oligomer "CBT" and will make it commercially available in limited quantities beginning this month.

When rendered into its cyclic form, PBT takes on a unique set of properties. For one thing, its viscosity drops dramatically. CBT has a viscosity of 150 cP at 320F and 20 cP at 380F—or about 5,000 times lower than the conventional form of the polymer. For another thing, its melt processes differently than an ordinary thermoplastics. While it does become fully molten at temperatures above 320F and flows to fill a mold, CBT also undergoes a chemical reaction more common to thermoset materials; catalysts added to the CBT before or during its processing trigger a non-exothermic reaction that repolymerizes the CBT into a semicrystalline solid PBT. The reaction works best at temperatures between 350 and 400F, and it can last less than a minute. Once the reaction finishes, the parts can removed from the mold while still hot, an important cycle time consideration. "Think of CBT as a material that processes easily like a thermoset but ends up as with all the advantages of a thermoplastic," says Steve Winckler, a Ph.D. chemical engineer and one of Cyclics' founders.

CBT's nature as a cross between thermosets and thermoplastics shows potential in three kinds of molding processes. Used in rotational molding, it expands the range of mechanical properties far beyond what polyethylene, the dominant material for that process, can do. CBT likewise adds capabilities to injection molding and compression molding. Dow Automotive, which has exclusive rights to Cyclics' technology for automotive applications, is leading the R&D charge for these last two processes. "We think it will be an important technology for painted body panels and for structural applications currently in steel," says Bob Rogowski, the company's director of new business.

Here's a closer look at CBT's design and manufacturing potential:

Rotational Molding

Rotational molding already has a lot going for it. Today, the process lets design engineers create feature-rich, hollow parts using relatively low-cost tooling. For jobs that don't require the production volumes or precision of injection molding, rotomolding can be a good way to go. But the process lacks in the materials department. "About 95 percent of the applications are in polyethylene," notes Michael Paloian, whose plastics design firm, Integrated Design Systems, works extensively with rotomolded parts. The process does use a smattering of nylon, too. But in general the mechanical property capabilities of the process come down to what linear low-density and high-density polyethylene can provide.

CBT could change all that. It turns out that the material works well in rotational molding. Winckler reports that Cyclics has run a number of test parts, and CBT produces parts whose mechanical properties far exceed those of polyethylene—no big surprise given standard PBT's property advantage. Winckler runs down the improvements as "higher tensile strength, higher modulus, and better creep performance." In fact, unreinforced PBT parts coming out of the tool have a flexural modulus and tensile strength about three times greater than the polyethylene materials that now dominate rotomolding. And CBT can be reinforced with glass to make its mechanical performance even better. Paloian adds that the CBT will likely resist warpage better than polyethylene, whose cooling dynamics and relative lack of stiffness make parts more likely to go out of whack.

The cosmetics of rotomolded parts may also improve. "We've produced parts with gloss levels close to a Class A surface," Winckler says, noting that the company has successfully painted the parts and molded in the color. And Paloian has noticed that the low-viscosity of CBT helps it replicate surfaces extremely well, a boon for applications needing molded-in labels.

Winckler predicts that the first applications for rotomolded CBT may utilize another useful property: Its resistance to fuel permeation. With permeation values at least 10 times better than polyethylene, CBT could work in smaller fuel tanks. Used alone its permeation advantage could drive down the wall thickness of the tanks. Or it could also be used in two-layer structures with lower-cost polyethylene.

For his part, Paloian has noticed only two potential property trade-offs. "Polyethylene has better impact performance," he says. "And the UV-stability of CBT is still a question mark."

Before it can be widely used in rotomolding, design rules for the material need to be developed. "Most of the part and tool design guidelines were written with polyethylene in mind," Paloian explains. But CBT behaves differently inside a tool. With a low melting temperature and its "water-like" viscosity, CBT coats the inside of a rotational molding tool better than polyethylene does, according to Paloian. This improved coating, in turn, will likely enable the creation of sharper, deeper, and finer features. "The rules have changed," Paloian says.

To see just how much they've changed, Cyclics has recently built what Winckler calls a "torture tool." It produces a test part designed by Paloian. Measuring 18 inches square by three inches thick, this hollow part, sports collection ribs, bosses, holes, and other features intended to push CBT to its limits. For instance, he designed in corners and rib fillets with much sharper radii than usual. He also specified distances between walls, aspect ratios of pockets, draft angles, and included angles on V-shaped features that would not work with polyethylene. "We broke all the rules associated with polyethylene," he says.

Injection Molding

CBT may not change injection molding to the same degree as it could change rotomolding. After all, ordinary PBT molds just fine already. But CBT does address a couple of niche opportunities.

One involves auto exteriors. Dow Automotive is looking at the material as a candidate for vertical body panels, which have to withstand high-temperature painting processes and also offer good dimensional stability. CBT looks like it could fit the bill—but not without some work. As Rogowski explains, unmodified CBT has a heat distortion temperature around 350F, not high enough to withstand e-coat ovens. But the materials' super low viscosity and re-polymerization process allows the incorporation of fillers that allow the polymer to handle oven temperatures to about 400F. "CBT allows you to rip open the chemistry of PBT to enhance its physical properties," he says. What's more, CBT doesn't absorb moisture the way materials solutions based on nylon can. "All the other solutions out there today contain some nylon, and with nylon you have to deal with the possibility of moisture growth," Rogowski says.

Dow engineers are currently just experimenting with injection-molded CBT. So far, it seems to process without any difficulties, Rogowski reports. Dow's remaining work involves quantifying the mechanical performance of finished parts, including a study of the material's CLTE. The company is also working to get a handle on the costs associated with CBT. Rogowski notes that Cyclics' first plant, in Schwarzheide, Germany, will start with 5.5 million pounds per year before ramping up to 11 million. The largest volume automotive applications may not become cost-competitive until Cyclics puts even more capacity on stream, which it plans to do in 2009 with a plant capable of producing 50 to 100 million lbs annually.

Aside from body panels, injection molded CBT may also be useful in applications that could benefit from a higher-flowing grade of PBT. Winckler says that some connectors fall into this category. "But you would really have to want that extra flow, because the cycle times would increase to account for the reaction," he explains.

CBT finally offers the potential to create tailor-made grades of PBT on demand. Winckler envisions using the barrel of the injection-molding machine as an in-line compounder. "CBT can be reacted in the presence of other materials and additives to create unique blends and alloys," he says. In this use of CBT, it would undergo its reaction before it entered the mold, rather than inside it.

Compression Molding

Though CBT winds up as a thermoplastic in the finished part, the material can also be processed using thermoset composite methods. Cyclics has demonstrated CBT with compression molding, RTM, vacuum bagging, and RIM processes to take a few examples.

For Dow Automotive, compression molded CBT has some compelling advantages in automotive applications. To begin with, CBT ends up as a thermoplastic that may have a more desirable mix of properties than thermosets, including recyclability. And Rogowski reports that Dow has been working with glass-reinforced CBT formulations that do a better job at balancing stiffness and toughness than vinyl ester composites, in part because of the excellent fiber wet-out from CBT's low viscosity. "The overall profile of stiffness and toughness is higher," he says. Exactly how much higher remains to be seen as Dow works to validate the mechanical properties of various CBT formulations.

CBT offers its share of processing edge, too. To begin with, it typically cures in less than 5 min in the structural applications under evaluation by Dow. And it cures without generating heat, eliminating the need to cool parts. CBT also lacks the VOCs of other thermoset composite processes. "A key enabler for this material is its processing advantages," he predicts.

Yet don't look for it to replace SMC anytime soon. Rogowski notes that SMC retains advantages in cosmetic appearance and cost. "I don't see CBT replacing SMC in the applications where SMC is already working," he says. "We see CBT as more of a metal replacement candidate that grows the pie for plastics."